We said that the smoking gun for human impact on climate
is the instrumental temperature record and the warming that it's shown since the
1970s, and that the models are able to explain that warming if you
tell them about rising greenhouse gas
concentrations, and they're not if you don't.
But that isn't really fair because it makes it seem like
the whole thing, our confidence in it, is based on the models.
And in fact, the reason why I personally
believe that the models know what they're talking about
comes from comparisons also with climate changes in
the deeper past, what are called paleoclimatology constraints.
if we look back further in the past, we have much larger changes in
Earth's climate to work with, but, unfortunately,
also, the uncertainties become bigger as well.
It's a trade off in that regard.
One of the most important findings
in paleoclimatology is the close link between atmospheric CO2 concentration
and ice volume and temperature going back through time
through the ice ages as revealed in the ice cores.
This plot, we'll talk more about in the coming lectures,
but it demonstrates a very tight correlation between CO2 and temperature.
Makes it clear that there is a CO2 impact on Earth's temperature.
And in fact, what we'll see is that we can quantitatively boil this down and try to
derive a value of the climate sensitivity. Delta T(2x), the amount of warming
that we expect from doubling CO2.
And we get pretty much the same answer from looking at these CO2
and temperature fluctuations as we do from
looking at the recent instrumental temperature record.
It gives us a separate independent check on the role of CO2 in
Earth's climate. Reconstruction
of Earth's climate in the past is based on what are called
proxy measurements of various climate parameters, mostly temperature
but also other things like water availability or or winds
or things like that can be pulled out from proxy measurements.
It's a proxy because we don't have a thermometer
from thousands of years ago to actually measure the temperature,
so we have to come up with other clever things
that we can use to guess what the temperature was.
And we do the best we can at that.
It's certainly not as good as having a real thermometer, but, we can, we
can get a lot of mileage by looking at things that we'll talk about.
Like the widths of tree rings of old fossil logs from ancient trees can tell us
about temperature and drought. Bore hole temperatures are temperatures
down inside an ice sheet or a a big rock or something like
that we can use to back out changes in temperatures in the past.
Basically, in general, a lot of the Paleo climate records come from ice cores that
accumulated in Antarctica or Greenland or on the tops of mountains little mountain
glaciers, and also sediment cores from the ocean and from lakes.
Let's hear about some paleo climate proxies.
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